Revegetation following soil disturbance in a California meadow: the role of propagule supply

Revegetation following a disturbance event initially should be constrained by the abundance and types of propagules available at the disturbed site. I tested this idea by conducting two experiments in which I created artificial soil disturbances by excavating or burying pre-existing grassland vegetation. In the first experiment, I varied disturbance intensity (depth), to investigate the consequences for revegetation when numbers of surviving propagules (dormant seeds and bulbs) were altered. In the second experiment, I varied the timing of disturbance, to investigate the consequences when disturbed sites experienced differing exposures to seasonal patterns of clonal growth and seed dispersal. I sampled these experiments from 1991 to 1993, and have interpreted their results using measurements of the seed bank, the bulb bank, and the seed rain. In the first (depth) experiment, bulbs declined in abundance with burial depth and were scarcer in deeper excavations. In contrast, numbers of annual graminoids initially showed no trends with respect to disturbance depth. These results reflect the depth distributions of the seed and bulb banks. Since bulbs occur deeply in the soil, progressively deeper disturbances left fewer survivors. Similarly, perennial graminoids could grow through the shallowest burials. In contrast, since the annual-graminoid-dominated seed bank is concentrated near the soil surface, disturbance depth mattered less to these species: any disturbance removing the surface layer was equally destructive. In the second (timing) experiment, more annual graminoids initially occurred in older plots. This result reflects seasonal patterns of seed production: plots exposed to more of the annual-graminoid-dominated seed rain supported higher densities of annual graminoids as a result. In subsequent years, the vegetation of most plots in both experiments was increasingly dominated by annual graminoids, again as a consequence of their great abundance in the seed rain. These results indicate that interactions between soil disturbances and sources of propagules play an important role in controlling early stages of succession in newly created gaps. They also suggest that disturbance may play different roles in communities characterized by species with different reproductive strategies. Understanding sources of colonists will improve our ability to predict the effects of disturbance.     

Revegetation following experimental disturbance in a boreal old-growth Picea abies forest

Abstract. We studied revegetation patterns after experimental fine-scale disturbance (e.g. uprooting) in an old-growth Picea abies forest in southeastern Norway. An experimental severity gradient was established by manipulation of the depth of soil disturbance; two types of disturbed areas were used. Species recovery was recorded in the disturbed patches in three successive years after disturbance. The cover of vascular plants and, even more so the cover of bryophytes and lichens, recovered slowly after disturbance. The least severe treatments (removal of vegetation and removal of vegetation and the litter layer) was followed by the fastest recovery. The mean number of vascular plant species was usually higher three years after disturbance than before disturbance, while the opposite was true for bryophytes. Several vascular plant species that were abundant in intact forest floor vegetation (Vaccinium myrtillus, V. vitis-idaea and Deschampsia flexuosa) recovered during a three-year period primarily by resprouting from intact rhizomes and clonal in-growth. Other important recovery mechanisms included germination from a soil-buried propagule bank (e.g. Luzula pilosa, Plagiothecium laetum agg., Pohlia nutans and Polytrichum spp.) and dispersal of propagules into the disturbed patches (e.g. Betula pubescens and Picea abies). Microsite limitation seemed to occur in several species that were abundant in the soil propagule bank (e.g. the ferns Athyrium filix-femina, Gymnocarpium dryopteris and Phegopteris connectilis) but which did not appear in disturbed patches. Disturbance severity influenced revegetation patterns, recorded both as trajectories of vegetation composition in a DCA ordination space and as change in floristic dissimilarity. The length of the successional path (compositional change measured in β-diversity units) increased with increasing disturbance severity, and was also influenced by the area of the disturbed patch and the distance to intact vegetation. The rate of succession depended on the method by which it was measured; decreasing year by year in floristic space, while first decreasing and then increasing in ordination space. The reason for this difference is explained.     

Succession patterns following soil disturbance in a sagebrush steppe community

Summary A study was begun in 1976 to measure succession patterns following soil disturbance within a sagebrush community in northwestern Colorado. The principal hypothesis was that type of disturbance affects the direction of succession, resulting in different plant communities over time. Successional dynamics were studied through 1988. Four types of soil disturbance resulted in 3 early seral communities: one dominated by grasses, one by annuals, and one intermediate. The annual-dominated communities were opportunistic on these sites, lasting 3–5 years and not determining the direction in which succession proceeded following their replacement. Twelve years after disturbance, 3 communities (one grass-dominated, one shrub-dominated, and one intermediate) occupied the site, the characteristics of which were functions of type of initial soil disturbance. For the period of time covered by this study (12 years), degree of disturbance was found to affect the direction of succession, resulting in different plant communities over time. There were, however, successional characteristics toward the end of the study that suggest that over a longer time period, succession might progress to a single community regardless of type of disturbance.     

Thirteen years of wetland vegetation succession following a permanent drawdown,Myrkdalen Lake,Norway

Myrkdalen Lake in Western Norway was subjected to a 1.4 mdrawdown in June 1987. Plant establishment and vegetation succession on theexposed sediments of a fluvial delta plain was monitored through 2000. Theinvestigated area extended from the original Equisetumfluviatile zone to the new lake edge. The substrate was homogeneousand consisted mainly of minerogenous fluvial sediments. Vegetation data weresampled within continuous quadrats along transects perpendicular to the shore,and they dropped 93 cm in elevation. Detrended CorrespondenceAnalysis and Canonical Correspondence Analysis confirmed that time sincedrawdown and elevation of the quadrats appeared to be ofnearly equal importance in explaining succession. Plant establishment was rapidon the exposed sediments due to a seed bank and to rapid invasion of plants.Thesuccession includes both floristic change as a function of time and a spatialseparation in relation to the water level. The species succession was marked bya growth form progression: mosses and annuals non-clonal perennialsclonal perennials. After one month, the annual Subulariaaquatica and small acrocarpous mosses dominated the site. Dominancethen shifted to Deschampsia cespitosa, Juncus filiformis, Blasiapusilla and Polytrichum commune. Subsequentlythere was an increase of Carex vesicaria in the lower zoneand Calamagrostis purpurea in the upper zone,while Phalaris arundinacea was common over most of theelevational gradient. Equisetum declined where it haddominated before drawdown, but it expanded gradually towards the new shoreline.The vegetation remained dynamic after 13 years and it is not considered to beinequilibrium with the new environmental gradient yet. However, annual changesmeasured by DCA scores have slowed and two vegetation zones have developed. Themajor vegetational differences along the elevational gradient can be explainedby the height of the mean June water limit. This example of species turnover inspace and time may be a model for other successions that occur along a stronggradient.     

The role of seed banks in vegetation dynamics and restoration of dry tropical ecosystems

Abstract. This paper reviews studies on seed banks in tropical dry vegetation and, for comparison, some wet tropical and dry subtropical vegetation. A first general conclusion is that tropical seed banks are smaller than those in temperate ecosystems. Many studies are devoted to only one species, among which are several Acacia species, both tropical and subtropical, some of which reach densities in the seed bank of up to 10000 seeds/m². The lesser importance of the seed bank strategy in tropical species may be related to higher risks of seed loss through higher mortality (fire, predation, pathogens etc.), or to intermittent germination occasions in relation to ephemeral favourable conditions (e.g. rains during the dry season). Regarding vegetation dynamics, the importance of seed banks and regeneration from seeds is most pronounced in larger forest gaps. Examples are discussed of rapid succession to forest of savannas after protection from fire, which is partly due to regeneration from the seed bank. The significance of seed banks in vegetation restoration projects is outlined and the need for additional sowing of seeds of important species underlined.     

The relationship between the soil seed bank and above-ground vegetation of a coastal barrier island

Abstract. The germinable soil seed bank is described from a coastal barrier island off the northwest coast of Florida, USA. Soil samples collected from seven vegetation types, recently deposited dredge spoil and unvegetated areas in autumn 1990 and spring 1991 were placed out in greenhouse trays. 110 taxa germinated from the samples with the largest number (41) being C₃ perennial dicots. The largest number of taxa germinated from dry (57) and wet (54) swales, the fewest (one species: Heterotheca subaxillaris) from strand. Similarity of seed bank densities to above-ground species cover was low (Jaccard's Index = 0.36), not different between vegetation types, but higher in the autumn than in the following spring. Compositional gradients in the seed bank and above-ground vegetation determined using DCA ordination were highly correlated and related to distance from mean high water, and plot elevation. At the landscape scale, the seed bank provided an equally clear delineation of vegetation types to that based upon the above-ground vegetation. The seed bank of low disturbance, late-succession vegetation types (wooded dunes, swales, marshes) was well developed (high species richness, emergent density, and percentage annual species) with the exception that the large-seeded woody species (i.e. Quercus spp.) were absent from the wooded dune seed bank. By contrast, a poorly developed and transient seed bank occurred in more frequently disturbed (extensive sand movement, salt spray), early successional dredge spoil, unvegetated areas and strand. These contrasts support a general pattern of increasing seed bank development and a persistent rather than transient seed bank with decreasing disturbance frequency, increasing time since disturbance and successional maturity.     

A vegetation change during a 20-year period following two continuous disturbances (mass-dieback of pine trees and typhoon damage) in the Pinus-Schima secondary forest on Chichijima in the Ogasawara (Bonin) Islands: which won, advanced saplings or new seedlings?

Three permanent plots (P1, P2, and P3) were established in the Pinus luchuensis-Schima mertensiana forest on Chichijima in the Ogasawara Islands. The complete process of the mass-dieback of the pine trees in the early 1980s and the consequences of the typhoon damage in 1983 were surveyed for 20 years (1980–1999) together with the vegetation recovery following these disturbances. The stem height and the growth condition of all the seedlings and saplings (P1: 1,632, P2: 1,681, and P3: 931) were recorded, and the relative light intensity and the weight of the A₀ layer were measured each year. Dead pine trees fell down and rotted gradually. In P1 and P2, the 1983 typhoon inflicted extensive damage to the canopy; the gap area ratio in the canopy was 60.1% and 58.1%, respectively. Trema orientalis seedlings emerged from buried seed concurrently in 1984, but they almost all died within a few years in P2. Many seedlings of S. mertensiana germinated in 1985 and 1986 in P1 and P2, and Psidium cattleianum (an introduced species) followed, especially in P2. Psidium attained a dominant position in the canopy of P2 as a result of the disturbances. In contrast, the structure and composition of the forest in P3, which consisted of emergent trees of Casuarina equisetifolia, did not change to any large extent. All of the individuals found in the plots, with the exception of the parent trees, were classified as advanced saplings (existing from 1980) and new seedlings (germinated during the survey period), and survival rates at 1, 10, and 20 years and growth following the disturbances were compared between them. By 1999, the advanced saplings, but not the new seedlings, overwhelmingly occupied the vacant spaces created in the canopy. Some seedlings became new advanced saplings following the disturbances. Based on these results, a two-step regeneration model that is related to the disturbances is proposed.     

Seed dispersal and vegetation development on a debris avalanche on the Ontake volcano,Central Japan

Seed dispersal and early revegetation processes were studied in an area devastated by a debris avalanche that occurred on volcano Ontake, Central Japan in 1984. The avalanche was initiated by a volcanic earthquake, and a 3.4 x 10⁷ m³ land mass destroyed the vegetation over 700 ha. The revegetation of the first five years was slow in the area of higher elevation (subalpine area). Both the speed of revegetation and the species richness of trapped seeds varied with the elevation. The post-disturbance vegetation on the deposit with surface soil or plant fragments was richer in species than that without any surface soil. Almost all of the seeds caught by sticky traps and most of the species with frequent occurrence in the vegetation after disturbance were wind dispersed. Wind dispersal was especially important on the deposit without surface soil or plant fragments.     

Long‐term compositional responses of a South African mesic grassland to burning and mowing

Question: What is the long‐term compositional response of grass and forb species to various combinations of burning and mowing? Can these responses be predicted from simple plant traits? Location: Ukulinga research and training farm (24°24′E, 30°24′S), Pietermaritzburg, KwaZulu‐Natal, South Africa. Methods: Grass species relative abundance in 1996 in various burning and mowing treatments of a long‐term (> 50 a) experiment was calculated from data obtained using a point sampling method, whereas forb species abundance in 1999 was determined using the importance score method. The experiment consisted of different frequencies (annual, biennial and triennial) of burning and mowing in winter or spring in combination with different frequencies of summer mowing (none, early, late or both). Results: Grasses responded to the type of disturbance (burn or mow) and frequency of burning, whereas forbs responded primarily to the presence or absence of any form of disturbance and secondarily to the timing of burning (spring versus winter). Summer mowing and annual or biennial dormant‐period burning maintained communities dominated by short grasses, whereas tall grasses dominated under annual dormant‐period mowing, triennial burning or protection from disturbance. Grass tillering strategy (below‐ or above‐ground) influenced response to burning frequency. Many erect herbaceous dicot species with aerial meristems were reduced in abundance by summer mowing whereas most small creeping herbaceous dicot species appeared to be dependent upon summer mowing. Conclusions: This long‐term experiment demonstrated that: (1) grasses and forbs responded differently to burning and mowing; (2) simple plant traits, such as height, position of tiller initiation, and position of meristems have potential for predicting the response of species to the timing and frequency of burning and mowing.     

山西桑干河流域湿地植被物种多样性研究

以109个样方调查数据为基础,采用丰富度指数、物种多样性指数和均匀度指数对山西桑干河流域湿地植物群落的物种多样性进行了研究,并用方差分析和多重比较的方法研究了各群落之间多样性指数的差异程度。研究结果表明：各植被类型的物种多样性指数大小排列依次为：柽柳—委陵菜群落＞柽柳+沙棘—鹅观草群落＞柽柳—鹅绒委陵菜群落＞柽柳—芦苇群落＞芦苇+水蓼+莎草群落＞芦苇+苔草群落＞百里香+铁杆蒿群落＞香蒲+水蓼群落＞水葱+轮藻+眼子菜群落＞藨草+莎草+泽泻群落＞水葱+藨草+芦苇群落＞狐尾藻+香蒲群落＞慈姑+芦苇群落＞水蓼+慈姑群落＞水莎草+水稗群落。对12种多样性指数进行比较后发现,R₀、H′和E₄优于其他指数。     

镇江内江湿地不同演替阶段植物群落小气候日动态

2005年5月,选择镇江内江湿地具有代表性的裸地、虉草群落和芦苇群落,分别代表植被群落的不同演替阶段,测定不同群落、不同层次的光照强度、气温、土温和空气相对湿度,研究其植物群落小气候的日动态.结果表明,随演替由裸地到虉草群落到芦苇群落进行,群落内光照强度、气温和土壤温度均明显降低,日变幅减小.其中,日均光照强度由1 204.7 μmol·m^-2·s^-1降至141.28 μmol·m^-2·s^-1,日均变幅由1 126μmol·m^-2·s^-1降至265 μmol·m^-2·s^-1;日均气温由32.2 ℃降至24.9 ℃,日均变幅由12.75 ℃降至4.8 ℃;日均土温由21.83 ℃降至19.47 ℃,日均变幅由4.5 ℃降至2.1 ℃.群落内空气相对湿度明显升高(由58.95%增至87.3%）,变幅减小(由29.75%降至5.15%）.生境具有早期的开放性和后期的封闭性,小气候环境朝着更为阴、凉、湿的环境变化,且波动性减弱,稳定性增强.各群落内的光强、气温、湿度及土温之间均存在一定相关,但不同演替阶段各因子间相关程度各异.     

An experimental test of the effects of inorganic sediment addition on benthic macroinvertebrates of a subtropical stream

Inorganic sediments of terrestrial origin may impact stream macroinvertebrate communities. Although input of terrestrial sediments to streams may occur naturally, human-induced activities in the catchment amplify this input greatly. We used an in-stream experiment to investigate whether short-term additions of terrestrial sediments of two size classes affected stream macroinvertebrates. The experiment was designed in blocks to minimize the influence of flow velocity and other environmental variables. Four treatments were employed: (i) addition of fine sand (0–0.24 mm), (ii) coarse sand (0.25–0.8 mm), (iii) fine+coarse sand, and (iv) control (water only). Macroinvertebrates were sampled immediately after the addition of sediments (or water). The experiment consisted of 20 blocks. We analyzed the response of the macroinvertebrate fauna in terms of abundance and species richness. Since species richness is strongly dependent on number of individuals sampled, we also analyzed rarefied species richness. Community structure was evaluated using a distance-based Manova on presence/absence and abundance data. The addition of coarse and fine+coarse sand reduced the abundance and species richness of macroinvertebrates in relation to the control. The response in terms of rarefied species richness in the treatments did not differ from the control, indicating that reduction in species richness was a sampling artifact resulting from decreased sample abundance. The Manova analyses indicated that coarse-sand addition caused changes in both species composition and community structure. Addition of fine and fine+coarse sand affected only slightly species composition and community structure. We concluded that even short-term input of terrestrial sediments causes impacts on benthic macroinvertebrates, and recommend that land-use management of tropical catchments should employ practices that reduce input of terrestrial sediments to streams. Handling editor: K. Martens     

Changes along a disturbance gradient in the density and composition of propagule banks in floodplain aquatic habitats

This study used germination methods to examine the density, species composition and functional composition of propagule banks in a series of riverine wetland aquatic habitats subject to varying degrees of hydrological and management-related disturbance. Under permanent inundation (the conditions prevailing at most sites during the growing season) propagule germination and species richness was low, with floodplain perennials and helophytes particularly affected. Densities of floodplain annuals were largely maintained through continued germination of a few flooding tolerant species. On damp mud (conditions associated with hydrological instability) total seedling number and species richness increased significantly, but species richness of germinating hydrophytes declined. Mean seedling density at 0–0.1m depth was 15450 ± 4400 m–2, reaching a maximum (162 050 m–2) in temporary backwaters. Annual (e.g., Lindernia dubia, Cyperus fuscus) and facultative ruderal species (e.g., Lythrum salicaria and Alisma plantago-aquatica) predominated. Vertical zonation of the propagule bank was weakly developed. The numbers of individuals and species germinating varied significantly between sites. The seasonal, most intensely disturbed sites (temporary backwaters) supported a numerically large, species-rich propagule bank based on floodplain annuals, while the permanent, less disturbed sites (ditches and an oxbow pond) had a small, species-poor propagule bank composed of hydrophytes and helophytes supplemented by allochthonous seed inputs. Sites intermediate on the gradient had a propagule bank dominated by facultative amphibious, ruderal hydrophytes. The composition of the seed bank and the established vegetation was most similar at the heavily disturbed sites where the seed bank was maintained by vigorously fruiting annuals and supplemented by inputs from temporary habitats upstream. At permanent sites much of the propagule bank composition could be accounted for by inputs of floodborne seed from the immediately adjacent floodplain. The established vegetation at such sites appeared to be maintained mainly by vegetative propagation with recruitment from the propagule bank likely only after severe disturbance. The potential contribution of functionally diverse propagule banks to sucessional processes within fluvially dynamic floodplain aquatic habitats is emphasised.     

A cumulative impact management plan for a forested wetland watershed in the Mississippi River Floodplain

A management plan using a watershed-scale approach was devised to limit loss of wetland functions in the one million ha Tensas Basin, Louisiana, U.S.A. Proposals to develop wetland areas are evaluated for their potential to affect the structure and function of the landscape as a whole. The plan required two prior steps. First, we assessed the structural and functional status of the landscape through time. Second, using the assessment, we formulated a set of environmental goals. The assessment indicated that the landscape is severely degraded; of the original forest, 85% has been lost, and the deforestation has negatively affected water quality and biota. Specific goals were devised to conserve remaining wetland resources and to restore functional integrity to the basin as a whole. On the basis of these two prior steps and principles of landscape ecology and conservation biology, we devised a plan that would establish two large tracts of bottomland forest (BLF) totaling 102 000 and 63 000 ha. These tracts would be established by reforesting about 1000 ha of corridors, primarily along streams, linking existing forest patches. In addition, set-back levees and man-made diversions would be incorporated to restore natural flooding to certain areas of remaining BLF. Existing wetlands would be prioritized on the basis of size and density of patches and placed in one of three management categories. Implementation of such a plan is possible under the present regulatory authority of U.S. federal government programs administered by regulatory agencies responsible for wetland protection.From a paper presented at the Third International Wetlands Conference, 19–23 September 1988, University of Rennes, France.     

广西阳朔岩溶植被演替过程种群变化及物种多样性

采用时空替代法研究广西阳朔岩溶植被在演替过程中的种群变化和物种多样性。研究表明,随着演替的进展,耐荫种群增加,乔木种群增加,优势种的更替呈现出一定的连续性,后一演替阶段的优势种往往已经隐含在前一演替阶段中;物种丰富度随演替进展表现出草丛<落叶阔叶林<灌丛<常绿落叶阔叶混交林的趋势;灌木层的物种丰富度在各演替阶段中均是最高的;在各演替阶段中,落叶阔叶林阶段的群落均匀度均表现为最大。